The proposed research seeks a better understanding of platelet adhesion which involves the interaction of platelets with the surface of an injured blood vessel. This interaction is an initiating and crucial step in normal hemostasis and in abnormal clotting processes that contribute to heart attacks and strokes, termed coronary and cerebral thromboses. Specifically, the research focuses on a complex of three platelet proteins that are required for platelet adhesion. These proteins, termed glycoprotein (GP) Ib, V, and IX, are present on the surface of human platelets and interact specifically with a circulating adhesive protein, von Willebrand factor (vWF), that becomes bound to the vessel wall. GPIb itself appears to play the major role in binding to vWF and in serving as an anchor to stop the circulating platelet at a site of vessel damage. The structure of GPIb is currently unknown, and the proposed research seeks to define GPIb at the level of its amino acid sequence and to define the structure of the mRNA(s) and gene(s) which code for GPIb. By means of molecular cloning techniques, the cDNA which codes for GPIb will be isolated and characterized. In addition, the GPIb gene(s) will be defined in terms of size, chromosomal location, regulatory features and coding regions. Both the mechanism by which GPIb binds to the adhesive protein, vWF, and the role of the other "complex" members, GPV and GPIX, are unknown. The binding reaction between GPIb and vWF will be studied at the level of the primary amino acid sequences involved in each protein. In addition, both GPV and GPIX will be purified and their contribution to the GPIb-vWF interaction will be defined. This research can provide insight into how platelets function in both normal and pathological clotting. The work can also provide insight into the genetic abnormality of patients who lack the GPIb complex.